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Networking Support In Java 2

Networking Support In Java 2. Fawzi Emad Chau-Wen Tseng Department of Computer Science University of Maryland, College Park. Overview. Networking Background Concepts Network applications Java’s objected-oriented view Java’s networking API (Application Program Interface).

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Networking Support In Java 2

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  1. Networking Support In Java 2 Fawzi Emad Chau-Wen Tseng Department of Computer Science University of Maryland, College Park

  2. Overview • Networking • Background • Concepts • Network applications • Java’s objected-oriented view • Java’s networking API (Application Program Interface) Last lecture This lecture

  3. Internet Design

  4. Internet • Network layer • Internet Protocol (IP) • Transport layer • User Datagram Protocol (UDP) • Transmission Control Protocol (TCP)

  5. Internet Protocol (IP) • Packet oriented • Packets routed between computers • Unreliable

  6. User Datagram Protocol (UDP) • Packet oriented • Message split into datagrams • Send datagrams as packets on network layer • Unreliable but fast • Application must deal with lost packets • Examples • Ping • Streaming multimedia • Online games

  7. Transmission Control Protocol (TCP) • Connection oriented • Message split into datagrams • Send datagrams as packets on network layer • Provides illusion ofreliable connection • Extra messages between sender / recipient • Resend packets if necessary • Ensure all packets eventually arrive • Store packets and process in order

  8. Transmission Control Protocol (TCP) • Reliable but slower • Application can treat as reliable connection • Despite unreliability of underlying IP (network) • Examples • ftp (file transfer) • telnet (remote terminal) • http (web)

  9. Client / Server Model • Relationship between two computer programs • Client • Initiates communication • Requests services • Server • Receives communication • Provides services • Other models • Master / worker • Peer-to-peer (P2P) Client Server Client Client Server Client

  10. Client Programming • Basic steps • Determine server location – IP address & port • Open network connection to server • Write data to server (request) • Read data from server (response) • Close network connection • Stop client

  11. Server Programming • Basic steps • Determine server location - port (& IP address) • Create server to listen for connections • Open network connection to client • Read data from client (request) • Write data to client (response) • Close network connection to client • Stop server

  12. Server Programming • Can support multiple connections / clients • Loop • Handles multiple connections in order • Multithreading • Allows multiple simultaneous connections

  13. Client / Server Model Examples

  14. Networking in Java • Packages • java.net  Networking • java.io  I/O streams & utilities • java.rmi  Remote Method Invocation • java.security  Security policies • java.lang  Threading classes • Support at multiple levels • Data transport  Socket classes • Network services  URL classes • Utilities & security

  15. Java Networking API • Application Program Interface • Set of routines, protocols, tools • For building software applications • Java networking API • Helps build network applications • Interfaces to sockets, network resources • Code implementing useful functionality • Includes classes for • Sockets • URLs

  16. Java Networking Classes • IP addresses • InetAddress • Packets • DatagramPacket • Sockets • Socket • ServerSocket • DatagramSocket • URLs • URL

  17. InetAddress Class • Represents an IP address • Can convert domain name to IP address • Performs DNS lookup • Getting an InetAddress object • getLocalHost() • getByName(String host) • getByAddress(byte[] addr)

  18. DatagramPacket Class • Each packet contains • InetAddress • Port of destination • Data

  19. DatagramPacket Class • Data in packet represented as byte array

  20. DatagramPacket Methods • getAddress() • getData() • getLength() • getPort() • setAddress() • setData() • setLength() • setPort()

  21. Socket Classes • Provides interface to TCP, UDP sockets • Socket • TCP client sockets • ServerSocket • TCP server sockets • DatagramSocket • UDP sockets (server or client)

  22. Socket Class • Creates socket for client • Constructor connects to • Machine name or IP address • Port number • Transfer data via streams • Similar to standard Java I/O streams

  23. Socket Methods • getInputStream() • getOutputStream() • close() • getInetAddress() • getPort() • getLocalPort()

  24. ServerSocket Class • Create socket on server • Constructor specifies local port • Server listens to port • Usage • Begin waiting after invoking accept() • Listen for connection (from client socket) • Returns Socket for connection

  25. ServerSocket Methods • accept() • close() • getInetAddress() • getLocalPort()

  26. Connection Oriented • TCP Protocol

  27. DatagramSocket Class • Create UDP socket • Does not distinguish server / client sockets • Constructor specifies InetAddress, port • Set up UPD socket connection • Send / receive DatagramPacket

  28. DatagramSocket Methods • close() • getLocalAddress() • getLocalPort() • receive(DatagramPacket p) • send(DatagramPacket p) • setSoTimeout(int t) • getSoTimeout()

  29. Packet Oriented • UDP Protocol

  30. URL Class • Provides high-level access to network data • Abstracts the notion of a connection • Constructor opens network connection • To resource named by URL

  31. URL Constructors • URL( fullURL ) • URL( "http://www.cs.umd.edu/class/index.html" ) • URL( baseURL, relativeURL ) • URL base = new URL("http://www.cs.umd.edu/" ); • URL class = new URL( base, "/class/index.html " ); • URL( protocol, baseURL, relativeURL ) • URL( "http", www.cs.umd.edu, "/class/index.html" ) • URL( protocol, baseURL, port, relativeURL ) • URL( "http", www.cs.umd.edu, 80,"/class/index.html" )

  32. URL Methods • getProtocol( ) • getHost( ) • getPort( ) • getFile( ) • getContent( ) • openStream() • openConnection()

  33. URL Connection Classes • High level description of network service • Access resource named by URL • Can define own protocols • Examples • URLConnection  Reads resource • HttpURLConnection  Handles web page • JarURLConnection  Manipulates Java Archives • URLClassLoader  Loads class file into JVM

  34. Java Applets • Applets are Java programs • Classes downloaded from network • Run in browser on client • Applets have special security restrictions • Executed in applet sandbox • Controlled by java.lang.SecurityManager

  35. Applet Sandbox • Prevents • Loading libraries • Defining native methods • Accessing local host file system • Running other programs (Runtime.exec()) • Listening for connections • Opening sockets to new machines • Except for originating host • Restricted access to system properties

  36. Applet Sandbox

  37. Network Summary • Internet • Designed with multiple layers of abstraction • Underlying medium is unreliable, packet oriented • Provides two views • Reliable, connection oriented (TCP) • Unreliable, packet oriented (UDP) • Java • Object-oriented classes & API • Sockets, URLs • Extensive networking support

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